The present invention relates to an amplifier comprising a differential stage having a first and a second input as well as a first and second output, the first and the second input being coupled through a first and a second capacitor to a first and a second input terminal, respectively, and the first and the second output being coupled to a first and a second output terminal, respectively, the input terminals being coupled through a first and a second feedback resistor to the output terminals.
Such an amplifier has already been described in French Patent Application 88 15958 filed on Dec. 6, 1988 by the Applicant. In this Application the output of the stage which drives the amplifier acts as a current source Io of given frequency, parallel to which is arranged a parallel self-capacitance filter Lo,Co. Two capacitors C and C' arranged in series at one of the inputs of the amplifier make it possible to eliminate the D.C. components without disturbing the filtering and to obtain a small reactive part at the input of the amplifier.
The value of the capacitors C and C' depends upon the value of the impedance parallel to the current source I.sub.o. In the case of a filter, too small a value disturbs the latter. In the case of a resistive impedance, too small a value induces an insufficient current coupling.